DSL parser, async DB worker, types, history, metadata, polish

Track 1 implementation plus polish round.

Parser (chumsky):
- Grammar-based DSL producing a typed Command AST.
- create table X with pk [name:type[,name:type...]] supports
  arbitrary names, any user type, compound PKs natively. Bare
  form errors with a friendly hint pointing at `with pk`.
- add column to table X: Name (type); drop table X.
- Required clauses use keyword grammar; -- reserved for opt-in
  flags (ADR-0009). Custom Rich reasons preferred when surfacing
  chumsky errors so unknown-type messages list valid alternatives.

Database (ADR-0010, ADR-0012):
- rusqlite + STRICT tables + foreign_keys=ON.
- Dedicated worker thread; mpsc Request inbox, oneshot replies.
- Typed DbError with friendly_message() hook for H1.
- Internal __rdbms_playground_columns metadata table preserves
  user-facing types across schema reads, atomically maintained
  alongside DDL via Connection transactions. list_tables hides
  it via the new __rdbms_ internal-table convention.

Types (ADR-0005, ADR-0011):
- All ten user-facing types: text, int, real, decimal, bool,
  date, datetime, blob, serial, shortid.
- Type::fk_target_type() for FK-side column-type rule
  (Serial->Int, ShortId->Text, others identity) -- foundation
  for the FK iteration.

App / Runtime / UI:
- update() stays pure-sync; runtime dispatches DSL via spawned
  tasks, results post back as AppEvent::Dsl*.
- Items panel renders live tables list; output panel shows the
  user-facing structure of the current table after each DDL.
- In-memory command history (Up/Down, draft preservation,
  consecutive-duplicate dedup) -- I2 partial.
- Mouse capture removed; terminal native text selection
  restored (toggle approach revisited when scroll/click
  features land).

Docs:
- ADRs 0009 (DSL syntax conventions), 0010 (DB worker),
  0011 (FK type compat), 0012 (internal metadata table).
- requirements.md progress notes; new V4 entry for the
  scrollable session-log + inline rich rendering + Markdown
  export direction.

Tests: 103 passing (91 lib + 12 integration), 0 skipped.
Clippy clean with nursery enabled.

src/action.rs

@@ -1,12 +1,20 @@
 //! Actions returned by the application's update function.
 //!
-//! `update` is pure with respect to the runtime: it mutates state
-//! in place and returns a list of `Action`s for the runtime to
-//! enact (e.g. quit the event loop). Side effects belong here,
-//! not in the update logic itself, which keeps `update` directly
-//! testable without a Tokio runtime or a real terminal.
+//! `update` is pure with respect to the runtime: it mutates
+//! state in place and returns a list of `Action`s for the
+//! runtime to enact (quit, dispatch a DSL command to the
+//! database, etc.). Side effects belong here, not in update
+//! itself, which keeps update directly testable without a Tokio
+//! runtime, a real terminal, or a database.
+
+use crate::dsl::Command;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Action {
+    /// Stop the event loop and exit cleanly.
     Quit,
+    /// Hand a parsed DSL command to the database worker. The
+    /// runtime executes it and feeds the result back as
+    /// `AppEvent::DslSucceeded` or `AppEvent::DslFailed`.
+    ExecuteDsl(Command),
 }